There are a number of indices for evaluating the performance of shock absorption systems in typical bicycle frames. For instance, the efficacy of shock absorption systems can be assessed based upon pedal energy loss, effect of braking on shock-absorption action, smoothness or comfort of shock absorption action, and pedal kickback caused by shock absorption action. In the shock absorption field, the term “kickback” refers to a phenomenon in which a change in chain length subjects the crank and pedals to the force of a rearward tension, thus, causing the rider to experience discomfort.
U.S. Pat. No. 4,039,200 discloses a conventional single swing arm mechanism (alternatively referred to as a “cantilever” system) for absorbing shock. To reduce pedal energy loss in this type of mechanism, the main turning point of a rear triangular frame relative to a front triangular frame is designed to be adjacent to where the chain connects with the front chainwheel. For example, if the front chainwheel cluster is a three-sprocket cluster, the main turning point is designed to be located between the intermediate sprocket and the small sprocket in the area where the chain connects. Alternatively, if the front chainwheel cluster is a two-sprocket cluster, the main turning point is designed to be located where the small sprocket connects to the chain. Finally, if the front chainwheel cluster is a single-sprocket chainwheel, the main turning point is designed to be located where this chainwheel connects to the chain.
This single swing arm system, however, suffers from several drawbacks. For instance, although this system can lower energy loss, it can result in greater pedal kickback, particularly on shock-absorbing frames with long ranges of motion. To reduce pedal kickback, the height of the main turning point must be lowered, thereby resulting in increased pedal energy loss. Moreover, in this system, when a rider is pedaling in a standing position, the compression of the shock absorber may change depending upon the amount of force applied by the rider. Therefore, it is not possible to adequately reduce energy loss during stand-up pedaling.
Other existing rear suspension systems pose similar problems. For example, the rear suspension system disclosed in U.S. Pat. No. 5,899,480 includes a four-bar linkage. In this system, the two turning points adjacent to the fork end are very close to one another, and a virtual pivot point (hereinafter “VPP”), which refers to a rotational axis of the center of the rear wheel during operation of the bicycle, is located slightly behind the pivot point at the front end of the lower fork. Since these points are located relatively close to one another, this system suffers from many of the same drawbacks as the single swing arm mechanism described above. In these systems, pedal energy loss and pedal kickback depend upon several factors. For instance, due to the height of the pivot point at the front end of the lower fork in these systems, it is not possible to achieve both low energy loss and low kickback. Moreover, the instantaneous center point of the rear wheel center relative to the front triangle is lower in these systems than the resultant force line when the rear brakes are applied. When the rear brakes are applied in systems with such a low instantaneous center point, the resultant force will stretch the rear shock absorber, thereby impeding the tension action of the shock absorber.
The system disclosed in U.S. Pat. No. 6,386,568 (“the '568 patent”) is also problematic in several ways. For instance, due to the relatively high location of the VPP, the pedaling force may generate tension on the shock absorber over a wide range of circumstances. In the system disclosed in the '568 patent, tension is distributed on the shock absorber when the rider pedals. The shock absorber thus may remain under constant tension. The shock absorber, however, is designed to act only when the force of impact is greater than the tensile force to which the shock absorber is being subjected. As a result, the shock absorber may not be able to absorb small impacts due to the constant tension exerted by the rider during pedaling. Hence, pedaling discomfort may occur.
Moreover, the use of a shock absorber with a “no-sag” setting in the system disclosed in the '568 patent fails to overcome the problem of pedaling discomfort. The term “no-sag setting” refers to a preset internal pressure exerted by the shock absorber that is designed to counteract against the pressure applied to the shock absorber by the body weight of the rider. When a shock absorber has such as no-sag setting, it will not compress when the rider is seated on the saddle.
In addition, the fact that the instantaneous center point of the rear triangular frame relative to the front triangular frame is way out in the front in the system disclosed in the '568 patent causes severe tension of the shock absorber when the rear brake is applied. This tension may further prevent the shock absorber from acting effectively during braking, thereby causing rider comfort levels to drop significantly. Finally, the tension exerted on the shock absorber in this system may unnecessarily reduce the useful life of the shock absorber.
U.S. Pat. Nos. 5,553,881, 6,206,397, and 6,488,301 also disclose four-bar linkage systems. In these systems, the rear wheel path is S-shaped, and the bottom half is a projecting, large chainwheel. During pedaling, the chain tension will pull the rear wheel to a certain point and, thus, achieve the effect of locking the shock absorption mechanism. If this point is designed as a rear wheel center position during a normal sag, then it may be possible to reduce pedaling energy loss. However, if ground surface impacts occur during pedaling, the ground surface impacts may not be effectively absorbed by the shock absorber due to constraints of chain tension. As a result, pedaling comfort may suffer. During stand-up pedaling, a weigh transfer effect may cause the rear wheel center to deviate from an optimal point. This may result in the shock absorption system being unable to effectively reduce energy loss during pedaling. In addition, variation of the distance between the multi-purpose axle connected to the seat tube and the center point of the rear wheel referred to as “RC”) variation can be considerable, thereby leading to pedal kickback problems.
In light of the drawbacks of conventional absorption systems, a need exists to provide a bicycle that has a rear suspension shock absorber system, which minimizes energy loss during pedaling in either a sitting or standing position, has minimal braking effects, has low pedal kickback, and is conducive to longer shock absorber life.